Any camera with a long telephoto lens can capture a pair of photos like the pair in the figure. For this pair, a Canon DSLR (digital single lens reflex) was used with a “lenses-only” 485 mm, f/5.5 telescope combined with a 2x teleconverter. This yielded an effective focal length of 970 mm, f/11.

The telescope was mounted on a tripod and a cable release was used to minimize vibration when triggering the shutter. A high ISO (sensitivity) setting allowed a short shutter time to freeze any motion due to mirror slap in the camera. Cameras are designed to assume everything reflects 18% of the light falling on it. That will tend to underexpose a full moon image since the Moon reflects only about 11% of the light. Bracket your shutter time to choose the most representative views later. The best exposure of the low Moon will differ from the best exposure of the high Moon.

To make your own comparison pictures, do not use a mirror-lens (catadioptric) telephoto or telescope or a pure reflecting telescope. The changing separation of the primary mirror and secondary mirror (and any correcting lenses), when focusing, changes the effective focal length of the telephoto. Temperature changes also affect the separation of the mirrors and even their curvature, changing the effective focal length. These mimic the effect of the observer’s distance changing with Earth’s rotation. Temperature changes affect refracting (lens) optics much less.

Supermoon Imaging
Which of these views of the Moon is a supermoon? Both! This is a size comparison of the Moon on the “supermoon” night of November 13-14, 2016. On the left the Moon was nearer the horizon, on the right the Moon was very high in the sky. Note the lower blue line cutting across the Moon’s south pole on the right image. The full frame of each picture is reproduced.

Figure Caption: Which of these views of the Moon is a supermoon? Both! This is a size comparison of the Moon on the “supermoon” night of November 13-14, 2016. On the left the Moon was nearer the horizon, on the right the Moon was very high in the sky. Note the lower blue line cutting across the Moon’s south pole on the right image. The full frame of each picture is reproduced. Photographs by S. J. Edberg

A careful look at the pair of images will show a slight difference in overall color of the Moon. The difference is due to the Moon’s altitude above the horizon when the photographs were taken. When the Moon was low, its light travelled through more air and slightly more violet-blue-green was absorbed from its reflected sunlight than when it was high in the sky. This gave the low moon a slight warm tint compared to the high moon. A similar effect is seen with the Sun. A red setting Sun is a great exaggeration of this effect. The same effects are also seen at sunrise and at moonset and moonrise.

An even more careful look will show that the phase of the Moon changed over the 6-hour separation of the images. This is most apparent along the grey zone in the upper left of the pair of SuperMoon images, around 10-11 o’clock (12 o’clock is at the top of the lunar images). Notice how more light colored areas, lunar highlands, have emerged in the right-hand image. This is due to the slight change in viewing angle (less than 2 degrees) as Earth rotated during the 6-hour separation of the images.

Using the SuperMoon made the most of the percentage change in camera-Moon distance as Earth’s rotation brought the camera closer to the Moon. The effect is reduced, if a more distant full moon is used for the comparison pictures.

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